In testing for conspecific activity as a possible Zeitgeber, one must often move the animal into a new environment (new cage) or part of its environment must be modified to allow for social contacts. The aim of this work was to determine whether modifications in the visual structural environment alone can influence the periodicity of the activity rhythm of Red‐winged Blackbirds(Agelaius phoeniceus).Six individually caged adult male birds were examined. Their free‐running period was measured under constant conditions in two different environments. Results clearly show that the periodicity remains stable as long as a bird is kept in the same environment, while it is shifted whenever the bird is transfered into an environment which presents a visually different structure. This source of variation should thus be considered more thoroughly in the interpretation of circadian rhythm experiments conducted under constant conditions of light and temperature.